Non-invasive evaluation of bone formation: measurements of serum alkaline phosphatase, whole body retention of diphosphonate and serum osteocalcin in metabolic bone disorders and thyroid disease.

Three noninvasive indices of bone formation, serum alkaline phosphatase (s-AP), 24-h whole body retention of diphosphonate (WBR), and serum osteocalcin (s-OC), the two lastnamed clearance-corrected, were compared in 121 patients with various bone disorders and in 50 patients with thyroid disease. In conditions with qualitatively normal matrix formation and mineralization, i.e. thyrotoxicosis, primary hyperparathyroidism, myxoedema and osteoporosis, the three indices deviated from average normal by about the same extent: 134%/128%/200%, 120%/113%/133%, 105%/100%/79% and 89%/86%/69%, respectively. A disproportionately marked deviation of s-AP was observed in states of abnormal matrix formation or mineralization, i.e. osteomalacia and Paget's disease: 430%/145%/282% and 348%/145%/202%, respectively. Furthermore, the formation indices correlate differently with s-calcium in hyper- and hypocalcaemic conditions. In primary hyperparathyroidism the respective r-values were 0.32/0.62/0.68, while an inverse pattern was observed in osteomalacia: -0.60/-0.51/-0.47. As very little is known about the secretion of AP and OC and their role in bone formation and mineralization, the cause(s) for the observed differences remain(s) uncertain.